When referring to MPEG-like signal what is meant is a compressed video signal of the general hierarchical form established by the MPEG Video Committee for International Organization for Standardization.
MPEG-like video compression cyclically provides fields/frames of data compressed by either intraframe techniques or interframe predictive techniques. Refer to FIG. 1 which shows a pictorial representation of the MPEG-like coding format. The frame sequence is merely representative. The letters I, P, and B above respective frames indicate the coding mode for the respective frame. The frame sequence is divided into groups of frames (GOF) each of which includes the same coding sequence. Each frame of coded data is divided into slices representing, for example, 16 image lines. Each slice is divided into macroblocks each of which represents, for example, a 16.times.16 matrix of pixels. Each macroblock consists of, for example, 6 blocks including four blocks of information relating to luminanee signal and two blocks of information relating to chrominance signal. The luminance and chrominance information are coded separately and then combined for transmission. The luminanee blocks include data relating to respective 8.times.8 matrices of pixels. Each chrominance block comprises an 8.times.8 matrix of data relating to the entire 16.times.16 matrix of pixels represented by the macroblock.
Blocks of data, encoded according to intraframe coding (I frames), consist of matrices of Discrete Cosine Coefficients. That is, respective 8.times.8 blocks of pixels are subjected to a Discrete Cosine Transform (DCT) to provide coded signal. The coefficients are subjected to adaptive quantization, and then are run-length and variable-length encoded. Hence respective blocks of transmitted data may include fewer than an 8.times.8 matrix of codewords. Macroblocks of intraframe encoded data, will include, in addition to the DCT coefficients, information such as the level of quantization employed, a macroblock address or location indicator, and a macroblock type.
Blocks of data encoded according to P or B interframe coding also consist of matrices of Discrete Cosine Coefficients. In this instance however the coefficients represent residues or differences between a predicted8.times.8 pixel matrix and the actual 8.times.8 pixel matrix. These coefficients are subjected to quantization and run- and variable-length coding. In the frame sequence I and P frames are designated anchor frames. Each P frame is predicted from the lastmost occurring anchor frame. Each B frame is predicted from one or both of the anchor frames between which it is disposed. The predictive coding process involves generating displacement vectors which indicate which block of an anchor frame most closely matches the block of the predicted frame currently being coded. The pixel data of the matched block in the anchor frame is subtracted, on a pixel-by-pixel basis, from the block of the frame being encoded, to develop the residues. The transformed residues and the vectors comprise the coded data for the predictive frames. As with intraframe coded frames the macroblocks include quantization, address and type information.
SUN and ZEDEPSKI in U.S. patent application Ser. No. 08/017,455 filed Feb. 12, 1993 and entitled APPARATUS FOR CONCEALING ERRORS IN A DIGITAL VIDEO PROCESSING SYSTEM (incorporated in its entirety herein by reference) describe a method of concealing errors in video data which was compressed in MPEG-like format. In this system the MPEG-like signal is arranged in transport packets for transmission. The transport packets include error codes for detecting loss or corruption of respective packets. Sun and Zdepski describe processes for concealing the effects of the lost transport packets on reproduced images. These processes include providing substitute compressed data and or providing substitute decompressed data dependent upon the type of compressed data lost or corrupted. Error concealment techniques set forth by Sun and Zdepski, include the substitution collocated blocks of decompressed data, substitution of non-collocated blocks of decompressed data derived from motion vectors from vertically adjacent macroblocks and the use of motion vectors from vertically adjacent macroblocks in forming substitute compressed data for a lost macroblock of interframe compressed data.
Intraframe compressed data tends to be of greater importance than the interframe compressed data in reproducing compressed images because all frames in a group of frames depend from or build on the decompressed intraframe data. Ironically error concealment of intraframes is least effective because there is less variety of encoded data from which to select substitute data. In particular, there are no motion vectors included within the intraframe coded information. As a result, substitute blocks of interframe data are substantially relegated to being either an average gray value or collocated blocks from a prior frame or interpolated blocks. If the blocks are collocated, moving image objects will create image artifacts. alternatively if the substitute blocks are interpolated, these areas will suffer loss of detail.